1. Technical Field
The technical field relates to a laser driving apparatus used in a laser light source for a display use.
2. Description of the Related Art
Conventionally, a laser device including a semiconductor laser unit that emits excited laser light and an optical wavelength conversion element of a nonlinear optical crystal that performs wavelength conversion of the excited laser light, as a fundamental wave, to laser light having a predetermined wavelength has been known.
In such a laser device using a nonlinear optical crystal, an optical output changes depending on the temperature of the nonlinear optical crystal. FIG. 1 shows a graph of characteristic showing a relationship between the temperature and optical output of a nonlinear optical crystal when the constant amount of light enters the nonlinear optical crystal from an excitation semiconductor laser unit. As shown in FIG. 1, there is an optimum temperature at which the optical output is maximum, and therefore temperature needs to be controlled such that the temperature of the nonlinear optical crystal reaches the optimum temperature.
However, the temperature versus optical output characteristic of the nonlinear optical crystal changes depending on usage environment or a change over time.
In a control method for controlling an optical output to be constant, taking into account such a change, control is performed such that, when the temperature of the nonlinear optical crystal is shifted from the optimum temperature, the output from the excitation semiconductor laser unit is increased. As a result, a driving current increases. In view of this, a technique of detecting the increase in driving current and controlling the temperature of the nonlinear optical crystal such that the detected value is a predetermined value is proposed (for example, Japanese Patent Application Laid-Open No. 2001-168439).
FIG. 2 is a block diagram showing a configuration of an example of a conventional laser driving apparatus.
In FIG. 2, reference numeral 1 denotes a laser device including semiconductor laser unit 11 as an excitation laser unit that emits excited laser light and nonlinear optical crystal 12 as an optical wavelength conversion element that performs wavelength conversion of the excited laser light, as a fundamental wave, to laser light having a predetermined wavelength. Reference numeral 2 denotes a driving section for causing semiconductor laser unit 11 to emit excited laser light. Reference numeral 3 denotes an optical detection section for detecting an optical output of the laser light whose wavelength is converted, exiting from laser device 1. Reference numeral 4 denotes an optical output control section for comparing an optical detection value outputted from optical detection section 3 with an optical output target value, to calculate a control value, and outputting the control value to driving section 2, whereby the optical output of the laser light exiting from laser device 1 reaches the optical output target value. Reference numeral 5 denotes a current detection section for detecting a laser driving current of driving section 2. Reference numeral 6 denotes an operating temperature setting section for finding a temperature setting value from the current detection value from current detection section 5. Reference numeral 7 denotes a temperature control section for controlling the temperature of nonlinear optical crystal 12 to match the temperature setting value found by operating temperature setting section 6. Temperature control section 7 includes a Peltier element (not shown) for heating and cooling nonlinear optical crystal 12 and a temperature detecting element (not shown) that detects a temperature of nonlinear optical crystal 12.
Next, in the laser driving apparatus shown in FIG. 2, an operation of controlling a temperature in nonlinear optical crystal 12 so as to maximize the optical output from laser device 1 will be explained.
First, upon the start of lighting, operating temperature setting section 6 provides the temperature T0 set at the end of a previous operation as an initial temperature setting value to temperature control section 7, to cause temperature control section 7 to start temperature control. At this time, optical output control section 4 operates such that laser device 1 generates a target optical output, and operating temperature setting section 6 stores the current detection value I0 current detection section 5 detects at this time. After an appropriate time has elapsed, operating temperature setting section 6 shifts the temperature setting value to a higher value by a micro-temperature ΔT and stores the current detection value I1 detected at the temperature setting value T0+ΔT. If I1≦I0, operating temperature setting section 6 sets the current temperature setting value T0+ΔT as a new temperature setting value. On the other hand, if I1>I0, then operating temperature setting section 6 shifts the temperature setting value to a temperature setting value T0−ΔT, which is lower than T0, and stores the current detection value I2 detected at the temperature setting value T0−ΔT. If I2≦I0, then the current temperature T0−ΔT is set as a new temperature setting value. On the other hand, if I2>I0, then operating temperature setting section 6 resets the temperature setting value to the initial temperature setting value T0. By the above-described operation, even when the temperature versus output characteristic of nonlinear optical crystal 12 changes over time, nonlinear optical crystal 12 can always operate at the optimum temperature.
A laser device composed of a nonlinear optical crystal is useful for, for example, a backlight source of a liquid crystal display. However, when the laser device is used in a backlight source of a liquid crystal display, it is necessary to adjust light dynamically.
However, when the laser driving apparatus shown in FIG. 2 is used in a backlight source of a liquid crystal display, there is a problem that, by frequently adjusting light for dynamic brightness adjustment, a current detection value changes and accordingly an optimum temperature setting value is not determined. Also, in a method of detecting an optical output and controlling a temperature such that the detected value is maximum, there is a problem that, when an optical output changes upon adjusting light, the maximum value of the optical output is not determined, and therefore control is difficult.
As methods of adjusting light of a laser device using an excitation laser unit and a nonlinear optical crystal, there are a method of changing a driving current linearly and a method of pulse width modulating (PWM) a driving current. From a viewpoint of an efficiency, PWM drive is more advantageous for the following reason.
In linear drive, the optical output from the nonlinear optical crystal is controlled by adjusting the driving current of the excitation laser unit. FIG. 3 shows a relationship between the input power and optical output of the laser device and a relationship between the input power and efficiency, for the case of the linear drive method. The optical output from the excitation laser unit is substantially proportional to the input power and the optical output from the nonlinear optical crystal is substantially proportional to a square of the optical output from the excitation laser unit, so that the optical output from the laser device increases in approximately proportional to a square of the input power and the efficiency increases approximately linearly relative to the input power. That is, the efficiency changes relative to the input power.
Next, FIGS. 4A to 4C show the relationships between an optical output and efficiency, and the driving current of the laser device in linear drive. The drive voltage of the excitation laser unit is substantially constant, and, consequently, the input power is substantially proportional to the driving current as shown in FIG. 4C. FIG. 4A shows the relationship between the driving current and the optical output. The optical output increases in approximately proportional to a square of the driving current and due to the influence of saturation of the optical output from the excitation laser unit, the optical output from the laser device gets saturated. FIG. 4B shows a relationship between the driving current and the efficiency. The efficiency increases in proportional to the driving current and due to the influence of saturation of the excitation laser unit, the efficiency decreases.
On the other hand, in PWM pulse drive, the average optical output from the nonlinear optical crystal is controlled by driving the excitation laser unit by a pulse current having a constant-amplitude and adjusting pulse duty. FIG. 5 shows the relationship between the average input power and average optical output of the laser device and the relationship between the average input power and efficiency, for the case of a PWM pulse drive method. The amplitude in PWM drive is constant, and so the average value is determined by duty. Consequently, the average optical output increases in proportional to the average input power and the efficiency is constant relative to the average input power. That is, the efficiency does not change relative to the average input power. In this way, in the case of PWM drive, the nonlinear optical crystal always operates at a high efficiency level.
As described above, in a case of operating a dynamic light adjustment, PWM drive is more advantageous from an efficient viewpoint, and therefore, the nonlinear optical crystal needs to operate at an optimum temperature further in PWM drive.
As shown in FIG. 1 showing the relationship between the temperature and optical output of a nonlinear optical crystal when the constant amount of light enters the nonlinear optical crystal from an excitation semiconductor laser unit, there is an optimum temperature at which the optical output is maximum, so that, as in the case of PWM drive, a temperature always needs to be controlled such that the optical output is maximum.